152 THE SPONGES. 



The oscula lead into main efferent canals which penetrate deeply into 

 the interior of the sponge, where they continue to be surrounded by a 

 thick layer of collenchyma. The canals are of about the same diameter 

 as the oscula. Some of them are, throughout the thickness of the sponge, 

 radially directed to the surface, but more are obliquely inclined, often 

 curving so that a section which is vertical to the surface of the sponge cuts 

 them transversely. This is the case in Fig. 4, Plate 23, which repre- 

 sents such a section taken through the loplwn and a part of the underlying 

 Gorgonia axis. 



The collenchyma surrounding a main canal in the sponge interior forms 

 a roughly cylindrical tract traversed by the canal, and preserving approxi- 

 mately the diameter which it has at the surface of the sponge. These 

 tracts cut the sponge body up into intervening regions permeated tlirough- 

 out their extent by the skeletal reticulum, which does not extend into the 

 collenchymatous tracts themselves. The parts of the body permeated by 

 the skeletal reticulum may be thought of as partitions between the collen- 

 chymatous tracts. These partitions more commonly have a thickness less 

 than the diameter of the collenchymatous tracts, appearing in sections as 

 thin trabeculae, as in Fig. 4, Plate 23. Elsewhere, however, in the 

 same specimen the skeletogenous partitions may appear as thick masses. 

 As regards the arrangement of the main canals and the relative disposition 

 of skeletogenous and collenchymatous tracts in the sponge interior, there is 

 much resemblance between this species and the reticulate specimens or 

 parts of specimens of I. lamella, but in the latter the main canals are more 

 frequently radially directed, and both the collenchymatous tracts and inter- 

 vening skeletogenous portions are thicker and probably on this account 

 appear better defined in sections (Plate 24, Figs. 3 and 4). 



Spicules. Megasdcres. I: Style, 220 /x x 14-16 /a, Plate 20, Fig. 5. 

 Spicule spinose with small, .sharp prickles, which are stronger and more 

 numerous near the ends. Extreme point smooth. Slightly curved, nearly 

 cylindrical, very slightly enlarged at rounded end and near the point. 



2. Subtylote, 220 /x x 8 ^jl, Plate 20, Fig. 6. Very slightly, scarcely at 

 all, curved. Tapering a little from the middle toward ends, which are 

 scarcely enlarged and most minutely spinose. Shaft in general smooth, but 

 near the ends are a few scattered prickles. 



Microscleres. 3. Bipocillus, 8 /.i long, Plate 19, Fig. 6 b, c, d, e. Curved 

 axis at the smaller end terminates in a spoon-shaped expansion with den- 



